Transmembrane proton translocation in the photosynthetic membranes of the purple bacterium Rhodobacter sphaeroides is driven by light and performed by two transmembrane complexes; the photosynthetic reaction center and the ubiquinol-cytochrome c oxidoreductase complex, coupled by two mobile electron carriers; the cytochrome and the quinone. This paper focuses on the kinetics and thermodynamics of the interaction between the lipophylic electron carrier ubiquinone-10 and the photosynthetic enzyme reconstituted in liposomes. The collected data were simulated with an existing recognized kinetic scheme [Shinkarev, V.P. &amp; Wraight, C.A. (1993) In The Photosynthetic Reaction Center (Deisenhofer, J. &amp; Norris, J.R., eds.), pp. 193-255. Academic Press, San Diego, CA, USA] and the kinetic constants of the uptake (7.2x10(7) M-1.s(-1)) and release (40 s(-1)) processes of the ligand were inferred. The results obtained for the quinone release kinetic constant are comparable to the rate of the charge recombination reaction from the state D(+)Q(A)(-). Values for the kinetic constants are discussed as part of the overall photocycle, suggesting that its bottleneck may not be the quinone uptake reaction in agreement with a previous report

Transmembrane proton translocation in the photosynthetic membranes of the purple bacterium Rhodobacter sphaeroides is driven by light and performed by two transmembrane complexes; the photosynthetic reaction center and the ubiquinol-cytochrome c oxidoreductase complex, coupled by two mobile electron carriers; the cytochrome and the quinone. This paper focuses on the kinetics and thermodynamics of the interaction between the lipophylic electron carrier ubiquinone-10 and the photosynthetic enzyme reconstituted in liposomes. The collected data were simulated with an existing recognized kinetic scheme [Shinkarev, V.P. & Wraight, C.A. (1993) In The Photosynthetic Reaction Center (Deisenhofer, J. & Norris, J.R., eds.), pp. 193-255. Academic Press, San Diego, CA, USA] and the kinetic constants of the uptake (7.2x10(7) M-1.s(-1)) and release (40 s(-1)) processes of the ligand were inferred. The results obtained for the quinone release kinetic constant are comparable to the rate of the charge recombination reaction from the state D(+)Q(A)(-). Values for the kinetic constants are discussed as part of the overall photocycle, suggesting that its bottleneck may not be the quinone uptake reaction in agreement with a previous report